Gases are typically compressed in compressors that are driven by electric motors. The compressors can be connected so that the gas is compressed in stages from ambient to a high delivery pressure. Interstage cooling is employed to remove the heat of compressor between stages. A common compressor design that is utilized is a centrifugal compressor. In a centrifugal compressor, the gas to be compressed enters an inlet and is compressed by action of an impeller that is rotated by the electric motor. The gas then passes through a diffuser and is discharged from a volute of spiral-like configuration.
In many industrial processes employing compressors, it is necessary to reduce or increase the flow rate of the gas to be compressed from a design level of flow rate. For instance, in cryogenic air separation, air is compressed in a series of compression stages, cooled to a temperature suitable for the rectification of the air and then introduced into a distillation column system to separate the air into its component parts, for instance, oxygen, nitrogen and argon. In order to optimize the margin in the sale of separated products, it is important that electrical energy costs be minimized. Since, the cost of electrical power will vary with the time of day, it is most cost efficient to conduct the air separation at off-peak times, for example during the evening. As such, during peak times, where energy costs are most expensive it can be profitable to turn the plant down by decreasing the flow rate of the air that is compressed.
The gas flow rate through a compression system can be controlled by an arrangement of inlet guide vanes that can be set from an open position to increasing more closed positions to impart a swirl into the incoming gas and thereby decrease the gas flow rate. More recently, high speed motors have become available that can be directly coupled to a compressor. Such motors also have a speed control that allows the speed of the compressor to be accurately controlled. This control of speed allows the flow rate of the gas to also be controlled in a more thermodynamically efficient manner than with the use of inlet guide vanes. An example of such a motor are permanent magnet motors having a variable frequency drive to control the speed. Such motors can be directly connected to the compressors or each of the compressors in a compression system.
There are certain limitations on the use of a speed control to control the flow rate of the gas to be compressed by the compressor. One major limitation concerns undesirable vibration modes within an intended operational speed range for the turbomachine. Since operating on or near these modes can cause rotor displacements that can cause damage to the rotor sufficient to render the motor inoperable, it is common practice to set restricted speed zones and avoid operation on or near these undesirable vibration modes. The width of such zones can render important operation ranges of the motor; and therefore, the compressor unusable.
As will be discussed, the present invention provides a method and apparatus for compressing the gas in which desired flow rates of the gas can be safely obtained over the entire operational range of the motor while the overall compressors thermodynamic efficiency is maximized.